344 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



During the periods of the Eopaleozoic and Neopaleozoic eras, the sedi- 

 mentary record of which in the continental basins is mostly accessible, snb- 

 mergent conditions predominated. In the Mesozoic and Cenozoic ages, 

 however, the marine sedimentary record of which is supposed to be largely 

 inaccessible, land or emergent conditions were dominant. It is to be ob- 

 served that in both diagrams — A showing average relief and B the maxi- 

 mum vertical displacement — the curve is accentuated between the Ordo- 

 vician and Silurian, the Tennessean and Pennsylvanian, and the Creta- 

 ceous and Eogenic, which respectively include the three periods of maxi- 

 mum diastrophic activity. That the average relief of the continents 

 at these times was actually greater than in respectively preceding and 

 succeeding periods is shown conclusively by the extraordinary amounts of 

 clastic sediments laid down in these ages. The same criteria, particu- 

 larly when the distribution of the clastic deposits is considered, indicate 

 also greater diversity of relief, a condition that is suggested again by the 

 fact that the unconformities at these boundaries are more clearly marked 

 than usual. (See "Gradational and Lithological Criteria," page 467.) 



Marine invasions from the four sides of the North American conti- 

 nent. — The principal oscillations of the strandline in Korth America 

 during the Paleozoic are shown graphically by the diagrams occupy- 

 ing pages 346 and 347. These indicate the direction and extent of a 

 large number of invasions of the continent by, respectively, Gulf of 

 Mexico, Boreal (chiefly by way of Hudson Bay), Pacific and Atlantic 

 waters and faunas. The east- west extent of the Pacific and Atlantic in- 

 vasions is indicated according to the location of easily understood longi- 

 tudinal topographic and geographic areas crossed by the respective waters. 

 The north-south extent of the Gulf and Boreal invasions is determined 

 by degrees of latitude, the area plotted being included between the 30th 

 and 75th parallels. When communication between the waters of two or 

 more of the oceans occurred, or when faunas that are provincially distinct 

 but so nearly of an age that they fall within a single time unit of the 

 rank discriminated on the charts, the approximate location of the result- 

 ing real or apparent intermingling of faunas is indicated by the letters 

 G = Gulf of Mexico, P = Pacific, A = Atlantic, and B = Arctic or 

 more accurately Hudson Bay.^^ The letter C indicates expansion of 

 Pacific waters northward or southward in the Cordilleran Basin. 



!''■ As stated on pp. 365 and 370, intermingling of faunas of distinct oceanic basins oc- 

 curred much less often than is suggested by our most detailed paleogeographic maps. 

 These maps are yet too synthetic to represent the facts of faunal distribution accurately. 

 Usually the indicated communication means no more than that the fauna of, say, the 

 Arctic type, invaded areas more commonly occupied by Gulf, Atlantic, or Pacific faunas, 

 and especially that such opposite and as yet undiscriminated invasions took place within 

 the time embraced in some paleogeographic map. 



